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A Split LGS/NGS Atmospheric Tomography for MCAO and MOAO on ELTs

A Split LGS/NGS Atmospheric Tomography for MCAO and MOAO on ELTs. Luc Gilles and Brent Ellerbroek Thirty Meter Telescope Observatory Corp. AO4ELT Conference Paris, June 22-26, 2009. Presentation Outline. Standard (integrated) tomography architecture for LGS MCAO and MOAO Formulation

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A Split LGS/NGS Atmospheric Tomography for MCAO and MOAO on ELTs

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  1. A Split LGS/NGS Atmospheric Tomography for MCAO and MOAO on ELTs Luc Gilles and Brent Ellerbroek Thirty Meter Telescope Observatory Corp. AO4ELT Conference Paris, June 22-26, 2009 AO4ELT, Paris 2009

  2. Presentation Outline • Standard (integrated) tomography architecture for LGS MCAO and MOAO • Formulation • Benefits and practical considerations • A split tomography architecture for LGS MCAO • Formulation • Benefits and practical considerations • A Strehl optimal split tomography architecture for LGS MCAO and MOAO • Formulation • Benefits and practical considerations • Comparative Monte Carlo simulation results for NFIRAOS AO4ELT, Paris 2009

  3. LGS MCAO and MOAO on ELTs • Under consideration/development for the E-ELT, GMT, and TMT • Demonstrators: MAD, Canopus, CANARY • Typical wavefront sensing requirements: • ~6-9 sodium LGSs for atmospheric tomography • ~3 low-order NGS WFSs • Sense tip/tilt and tilt anisoplanatism errors • Sense focus errors due to sodium layer range variations • Standard approach to tomographic wavefront reconstruction: • Minimal variance estimation + least squares DM fitting • LGS and NGS measurements concatenated into a single vector • “Pseudo open-loop” measurements used AO4ELT, Paris 2009

  4. Standard (Integrated) Control Architecturefor LGS MCAO and MOAO “Open-loop” LGS Gradients Tip/Tilt and Diff. Focusremoval Modal Projection and Servo filtering Minimal Variance Atmos Tomo DM fitting Concatenate DM/TT commands “Open-loop” NGS Gradients (~12) Low Pass Filtering AO4ELT, Paris 2009

  5. Benefits and Practical Considerations • Strehl optimal in the limit of accurate tomographic solution • Applicable to both MCAO and MOAO • NGS and LGS WFS measurements are very different: • NGSs are typically faint, and measurements require pre-filtering to optimize servo compensation • Requires efficient joint estimation of both low- and high-order atmospheric modes • Impacts tomography algorithm (choice of “solver,” number of iterations, memory …) • Tomography step mixes LGS and NGS WFS operators • Impacts practical implementation of ray-tracing Split LGS/NGS architecture preferred AO4ELT, Paris 2009

  6. A Split Tomography Control Architecture for LGS MCAO Modal Projection and Servo filtering • NGS-controlled modes are invisible to tip/tilt-removed LGS WFSs • Consist of Tip/Tilt and 3 “cancelling” quadratic modes on 2 DMs Minimal Variance LGS Atmos Tomo DM fitting “Open-loop” LGS Gradients Tip/Tilt and Diff. Focusremoval LGS DM commands Least-Squares Rank-5 Modal Reconstruction Closed-loop NGS Gradients (~12) NGS DM commands Servo filtering TT commands AO4ELT, Paris 2009

  7. Benefits and Practical Considerations • Tomography step contains only LGS operators • Relaxes computational requirements • Separate NGS servo compensation in 5 modes • NGS reconstruction and servo compensation easy to update for each new NGS asterism • Simple NGS reconstruction/control model requires good LGS correction to minimize aliasing of LGS DM commands into NGS loop (may impact sky coverage) • Applicable to MCAO, inappropriate for MOAO (oversimplified definition/control of NGS modes) AO4ELT, Paris 2009

  8. A Strehl Optimal Split Control Architecture for LGS MCAO and MOAO • Concept derived from standard (integrated) tomography by application of the Sherman-Morrison matrix inversion formula • Analytically equivalent to integrated tomography in the limit of an exact tomography matrix system solution • NGS modes dependent upon NGS asterism (location and magnitudes) and seeing • Must be pre-computed accurately and updated at ~0.1 Hz • A practical approach has been defined AO4ELT, Paris 2009

  9. Benefits and Practical Considerations • Robust to LGS/NGS loop cross-coupling • Practical, similar to previous split MCAO control architecture • Strehl optimal in the limit of accurate LGS tomographic solution • Applicable to both MCAO and MOAO • Cost of NGS reconstruction dominated by background computation of NGS modes AO4ELT, Paris 2009

  10. Sample NGS Mode Distortion Patterns for NFIRAOS AO4ELT, Paris 2009

  11. Comparative Performance Evaluation for NFIRAOS in the high SNR Regime • 4 NGS asterisms of 16th magnitude • Common 800 Hz sampling of LGS and NGS loops • Simulated NGS WFSs: Z-Tilt • NGS reconstruction: Z-Tilt WFS model • Simulated LGS WFSs: physical optics with short-exposure matched filters • Tomography algorithms: CG30 and FD3 (split, new split, integ)

  12. Sample Split Tomography Performance • Median seeing • Includes 116 nm RMS in quadrature of implementation errors • 15 arcsec FoV averaged WFE • 2400 frames averaged; single turbulence realization AO4ELT, Paris 2009

  13. Sample Comparative Performance AO4ELT, Paris 2009

  14. Summary and Plans • A split wave-front control architecture has been introduced for LGS MCAO • NGS reconstruction and servo compensation easy to update for each new NGS asterism • Requires good LGS correction to limit aliasing into NGS loop • Applicable to MCAO, unsuitable for MOAO • A Strehl optimal split control architecture has been developed for LGS MCAO and MOAO • Practical, similar to previous split LGS MCAO architecture • Applicable to both MCAO and MOAO • 35-60 nm RMS improvement for sample asterisms in the high SNR regime for NFIRAOS • Detailed sky coverage simulations planned in near-future • MOAO analysis planned at completion of MCAO analysis

  15. Acknowledgements • The work is supported by the TMT project. The authors gratefully acknowledge the support of the TMT partner institutions. They are: • the Association of Canadian Universities for Research in Astronomy (ACURA) • the California Institute of Technology, and • the University of California • This work was supported as well by • the Gordon and Betty Moore Foundation • the Canada Foundation for Innovation • the Ontario Ministry of Research and Innovation • the National Research Council of Canada • the Natural Sciences and Engineering Research Council of Canada • the British Columbia Knowledge Development Fund • the Association of Universities for Research in Astronomy (AURA) • and the U.S. National Science Foundation AO4ELT, Paris 2009

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